Producing plastic products by blow molding is well known. One way of doing this is to provide mating mold halves having shells defining the shape of the desired product to be molded nested within mold holders. A parison is provided inside the mold halves and air is then introduced into the parison to expand it to conform to the shape defined by the mold shells. Upon cooling to a suitable temperature, the molded product is then removed from the blow mold and thereafter, employed in its desired capacity.
Molds are normally made of aluminium. The mold surfaces, which define the shape of the product to be blown, require machining and hand finishing or polishing in order to produce a desired surface finish. Even when the mold surface has been machined, it is difficult to create fine detail which will be easily reproduced in the subsequent blown product. An example of this fine detail desired would be a very small symbol indicating the product is made of recycled plastic, or intricate designs indicating the proprietor's trade-mark. This machining and polishing requirement, apart from being complicated, makes it expensive to produce the molds.
The blow molding process requires that the blow mold unit be cooled or heated to suit the process. Until the blow mold unit has been sufficiently cooled or heated, the blown product cannot be removed. Pre-mature removal of the blown product is likely to result in deformation of its shape which in turn, may affect its commercial applicability as well as its aesthetic appeal. Such cooling or heating is normally accomplished by introducing cold or hot liquid respectively, through a network of heat transfer passages that are machined into the molds. Again, this makes the molds complex, and expensive to produce.
An example of a liquid cooled mold is shown in U.S. Pat. No. 3,768,948, issued to Horberger Jr. et al. This patent shows a blow mold consisting of a carrier and a shell, the shell having fins on its back surface extending toward the carrier, and fin channels provided by the carrier, thus creating a complex fluid cooling network for passage of heat transfer fluid therethrough. The precision manufacture required for the shell and carrier combinations of this nature is expensive. Proper alignment and cooperation of the fins and fin channels in manufacture is also complex.
Other examples of liquid cooled molds produced in the past are shown in U.S. Pat. No. 4,072,456 issued to Appel et al. and U.K. Pat. No. 2,240,300 issued to Hanna et al. Again, the molds are complex and expensive to produce.